In the (petro)chemical industry, alkaline solutions (e.g. sodium hydroxide) are typically used to remove sulfur compounds (e.g. H2S) from hydrocarbon streams. This is typically done in a sulfide scrubber. Although this method is very effective in removing sulfur compounds, it results in an alkaline stream with a pH of more than 12 that is contaminated with large amounts of sulfide (S2−) and smaller amount of other compounds (e.g. mercaptans, BTEX, and phenols). These contaminated alkaline waste streams are referred to as sulfidic spent caustics. Traditionally, sulfidic spent caustic streams are treated by means of wet air oxidation. During wet air oxidation all reduced sulfur species are oxidised with air to sulfate and/or thiosulfate. Furthermore, the other compounds (e.g. mercaptans, BTEX, and phenols) are also oxidised to form carbon dioxide (and more sulfate and/or thiosulfate). Although wet air oxidation can be very effective in treating the sulfidic spent caustics, it is only so at high temperatures and pressures, which means that the energy costs and operational costs of the process are high. Furthermore, wet air oxidation does not allow the reuse of the alkaline solution as the oxidation of the sulfur species to sulfate and/or thiosulfate results in the production of acid and loss of alkalinity.
Sulfidic spent caustics can also be treated by means of membrane electrolysis as described in WO99/34895 (=U.S. Pat. No. 6,132,590). According to this process, the oxidisable sulfur species (e.g. sulfide and mercaptans) are oxidised at the anode in the anode compartment of an electrolysis cell. This oxidation reaction produces oxidised sulfur species (e.g. sulfur, sulfate and disulfides). The anode compartment is separated from the cathode compartment by means of a cation exchange membrane. When an electrical current flows, this membrane transports cations (e.g. sodium ions) from the anode to the cathode compartment. The cathode compartment is continuously fed with deionised water. In the cathode compartment the sodium ions from the anode compartment combine with hydroxyl ions produced in the cathode reaction. This turns the deionised water into an alkaline stream. This alkaline stream can be reused in the caustic scrubber.
Unfortunately, this process has several disadvantages. First of all, there is a severe risk of sulfur accumulation on the electrode causing an increase of the electricity consumption by the electrolysis cell due to an increase of the anode resistance. Secondly, not all the sodium ions can be retrieved from the influent as that would cause a strong increase of the anolyte resistance (if all the sulfide is converted to sulfur). This increase of the anolyte resistance would again increase the electricity consumption to unacceptable levels. If not all sodium ions can be retrieved, however, a make-up sodium hydroxide stream is required to operate the caustic scrubber. Thirdly, if the sulfur accumulation can be prevented by converting all sulfide to sulfate, a strong acidification of the effluent can be expected that requires neutralization. This unfortunately consumes a large part of the produced alkaline. Finally, the remaining solution will require additional treatment as it will still contain disulfides and sulfate.
Alternatively, the sulfidic spent caustics can be treated through the biological oxidation of sulfide by aerobic Thiobacilli as described in WO98/04503. This oxidation can be represented by the following equations:HS−+0.5O2→S0+OH−  (I)HS−+2O2→SO42−+H+  (II)
In reaction (I) sulfur is formed with an increase of pH, whereas in reaction (II) sulfate is formed with a drop in pH. As can be seen from reaction (I), the alkalinity in principle can completely be recovered if the biological oxidation completely proceeds through equation (I), which means that theoretically the treated stream can be reused as alkaline scrubber liquid and the produced sulfur can be reused (e.g. for sulfuric acid production, as a fungicide, as a soil fertiliser, etc). A drawback of this system, however, is the fact that the micro-organisms can be severely inhibited by the other compounds (e.g. mercaptans, BTEX, and phenols) that are also often present in sulfidic spent caustic streams. Furthermore, this method can only be reliably applied at pH values below 10.
This means that a large part of the sulfide needs to be converted to sulfate (instead of sulfur) according to reaction (II) in order to lower the pH. Sulfur normally occurs as a suspended solid in the water phase and, therefore, it can be easily separated from the treated stream and reused. Sulfate, however, will remain in solution and will accumulate when the stream is recycled for reuse in the sulfide scrubber. Furthermore, a pH of below 10 makes reuse of the treated stream as alkaline scrubber liquid unfeasible as the sulfide scrubbers often need to be operated at pH values above 12.